Mosquito-borne viruses are of increasing concern in the US and Florida. In 2002, there were over 4000 human cases of West Nile (WN) fever or encephalitis in the US. Other arboviruses such as St. Louis encephalitis (SLE) virus, Eastern equine encephalitis; and Western equine encephalitis viruses also cause human disease and are of concern. Control and reduction of cases requires a clear understanding of the transmission cycle and factors, which influence transmission, but much of the needed information is lacking. In Florida, Culex nigripalpus and Cx. quinquefasciatus are considered the most important vectors of WN and SLE viruses. We will study these vector-virus systems using an integrated approach with field, laboratory and theoretical methods. Our focus is on 3 specific aims: 1) to understand the relationship between infection and transmission rates in mosquitoes; 2) to consider the effect of mosquito population age structure on transmission cycles; and 3) to determine the impact of multiple vector species on arbovirus systems. Field studies will use a modified sentinel chicken protocol and aspirator collections to determine the relative abundance and age structure of several populations of the two Culex species and their infection and transmission rates. Laboratory studies will focus on the effect of temperature, age, and blood meal on infection and transmission rates. Mathematical studies will integrate available information and the results of the laboratory and field studies to investigate the relationship between infection and transmission rates and the effect of age structure and multiple vectors on transmission cycles. It is critical to use an integrated approach to study arbovirus cycles, as they are complex systems that cannot be understood by considering isolated elements. We will use integrated lab, field and modeling studies to investigate SLE and WN virus cycles in Florida. The results will be applicable to other viruses and other locations, and will provide information on the interpretation of surveillance results, the influence of mosquito age and population age structure on transmission, and the effect of multiple species of vectors involved in the transmission cycles.